The present invention relates to electrical load control systems in which a plurality of loads are selectively and recurringly connected to and disconnected from a power source in order to regulate the number of loads that are energized by such source in accordance with a predetermined regulation objective.
Electrical load regulation means are utilized in a wide variety of control systems where it is necessary or desirable to control the number of loads that are energized at any given time. A particular example of such regulation is found in systems for automatically, selectively deenergizing (shedding) electrical loads, in an installation including a predetermined plurality of loads, in order to reduce the rate of electrical energy consumption. The reduced consumption may be necessary in order to stay within predetermined consumption limits established by the utility company that supplies the power and/or by the utility customer. In an effort to increase the efficiency of electrical energy use by the utility customers, the utility industry typically charges users according to a formula based upon actual power consumption and the maximum energy demand, contracted for or set by the user, for each of a continuous succession of predetermined demand intervals, usually 15 or 30 minutes per interval. To reduce the expense associated with the maximum demand factor in the billing, it is a benefit to the customer to operate his plant to utilize up to or just below the maximum demand limit within each demand interval.
Various load regulation systems have been devised for automatically maintaining the customer's power consumption within the predetermined demand limit. Briefly, these systems operate to monitor the power consumption during the demand interval, continually comparing it with a predetermined reference rate of use, and automatically disconnecting (shedding) and reconnecting (adding or restoring) loads as required in order to maintain a predetermined relationship to the reference consumption rate. The reference rate, when projected to the end of the demand interval, maintains the customer within the maximum demand limit. Examples of such regulation systems are disclosed in U.S. Pat. Nos. 3,296,452 issued to Wallace L. Williams, Jan. 3, 1967, and 3,505,508 issued to W. L. Leyde on Apr. 7, 1970. In these systems, a predetermined group of loads, for example 10 loads, are initially all turned on at the commencement of each demand interval, and thereafter, if necessary, selectively shed in order to stay within the predetermined demand rate. Also, if less than all of the loads are energized, and the actual consumption drops significantly below the reference rate, then one or more of the previously shed loads is added or restored to the previously energized status.
Other load regulation systems in which a plurality of loads are selectively added and shed with respect to an energizing source include equipment which produces a regulatable output in response to the combined operations for a plurality of individual electrical load components. For example, a pumping system may provide an output flow regulated by turning on and off one or more of a plurality of pumps. Other examples are electrical furnaces having a plurality of individually energizable heating elements and in which the heating output of the furnace is to be regulated, and an air conditioning system having a plurality of individually controllable fans.
In the foregoing control systems, it is desirable to provide for distributing or sharing the on and off times independently demanded by the regulating means, among all of the predetermined plurality of electrical loads. For example, an industrial plant or commercial building may include a plurality of air conditioning or heating units distributed throughout the facility, where each unit constitutes one of a plurality of electrical loads subject to being intermittently shed by the load demand regulation means. In many installations, there is no particular ranking of relative importance of the units, such that each unit may be interrupted for brief periods of time without seriously disrupting the overall use or operations of the building. Nevertheless, available load control systems are provided with built-in load control priority schemes, which are either hard wired into the system, or manually selectable by switches, jumpers, or other manually effected controls. Accordingly, certain of the plurality of loads, such as the air conditioning units in the above example, will be designated as low priority units subject to frequent interruption of service. Although it is possible to manually change the priority scheme, this is impractical in that it requires the attention and time of an operator to monitor the system and periodically change the controls.
Another disadvantage of the functioning of existing demand controllers is the interruption of one or more low priority loads for undesirable long periods of time. This may occur in a control system in which the loads, when all connected, consume power close to or just slightly in excess of the allowable rate of consumption. Accordingly, after the commencement of each demand interval, one or more of the lower priority loads is turned off and remains off until the end of the interval so as not to exceed the maximum demand. Thus, a situation may result in which one or more of the loads, such as the air conditioning units in the above example, are out of service for a significant amount of the time.
In the case of control systems in which loads are selectively turned on and off at reoccurring times in order to regulate an output condition, as in the multiple pump example above, it is desirable to rotate or vary the sequence in which the loads are energized. Such rotation equalizes the wear among the various loads and results in a longer, trouble free life of the system. While certain methods and apparatus for sequentially energizing and/or alternating loads are available, they have not provided the reliability, long operating life and versatility that is desired in many applications.